I want to do a lesson about basic manners related to grammar, for example not saying "open the door" but using "could you open the door?" and "would you open the door please."

What other things should I cover related to grammar and language in general?

Some other things I should cover
-Yelling "hey" to people can also be weird.
-Ma'am and sir
-Using "i'm afraid" for giving bad news
-Not saying words like retarded. I forgot what this was called in linguistics, but using a word with the same meaning but better feeling, i.e. "collateral damage."

Also I notice in some situations it sounds better to use "could" than "would" (and vice versa). Is there a general rule?

-Not saying words like retarded. I forgot what this was called in linguistics, but using a word with the same meaning but better feeling, i.e. "collateral damage."

I think you're looking for "euphemism," but "retarded" is something else--it was once the preferred and official term, but has been replaced (twice, I believe..."developmentally disabled" was the correct term for a while, but I'm pretty sure that one's also been replaced...)

My gut instinct is that I would teach euphemisms (disabled, challenged, retarded, etc.) in a different lesson than one on etiquette, so as not to confuse students. They are related, but I think a Korean would easily see the latter as a matter of grammatical politeness between different age or prestige levels, and euphemisms as a complex and more political matter. Not everyone agrees in the west on these wordisms, thus they keep changing, and you will have to make that distinction.

I would have the students imagine or role-play ordering in a restaurant or hot dog stand, or something, rather than giving them a list of modal auxiliaries. This way you can show how "gimme a hot dog," "I want a hot dog," and "may I please have a hot dog" are different. You can move from there to other examples.